Augmenting a Webmail Application with Cryptographic Puzzles to Deflect Spam

In order to increase the resilience against spam, the authors design and implement a protocol based on cryptographic puzzles for an open-source web based e-mail client. Their proposal is compatible with existing e-mail infrastructure and does not require modifications on the server side. The only add-on is a stand-alone ticketing server that is used to deliver the current cost to each sender. The puzzles that they use are time-lock puzzles which have the benefit that they cannot be subject to a parallel attack due to their intrinsic sequential nature.